Method and apparatus for detecting an end-point in chemical mechanical polishing of metal layers

ABSTRACT

An apparatus, as well as a method, determines an endpoint of chemical mechanical polishing a metal layer on a substrate. The method of the apparatus includes bringing a surface of a substrate into contact with a polishing pad that has a window; causing relative motion between the substrate and the polishing pad; directing a light beam through the window, the motion of the polishing pad relative to the substrate causing the light beam to move in a path across the substrate; detecting light beam reflections from the substrate and a retaining ring; generating reflection data associated with the light beam reflections; dividing the reflection data into a plurality of radial ranges; and identifying the predetermined pattern from the reflection data in the plurality of radial ranges to establish the endpoint.

BACKGROUND

[0001] The present invention relates generally to chemical mechanicalpolishing of substrates, and more particularly to methods and apparatusfor detecting an end-point of a metal layer during a chemical mechanicalpolishing operation.

[0002] An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive or insulative layerson a silicon wafer. After each layer is deposited, the layer is etchedto create circuitry features. As a series of layers are sequentiallydeposited and etched, the outer or uppermost surface of the substrate,i.e., the exposed surface of the substrate, becomes increasinglynon-planar. This non-planar surface presents problems in thephotolithographic steps of the integrated circuit fabrication process.Therefore, there is a need to periodically planarize the substratesurface.

[0003] Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is placed against a rotating polishing pad. Thepolishing pad may be either a “standard” pad or a fixed-abrasive pad. Astandard pad has a durable roughened surface, whereas a fixed-abrasivepad has abrasive particles held in a containment media. The carrier headprovides a controllable load, i.e., pressure, on the substrate to pushit against the polishing pad. A polishing slurry, including at least onechemically-reactive agent, and abrasive particles if a standard pad isused, is supplied to the surface of the polishing pad.

[0004] One problem in CMP is determining whether the polishing processis complete, i.e., whether a substrate layer has been planarized to adesired flatness or thickness. Variations in the initial thickness ofthe substrate layer, the slurry composition, the polishing padcondition, the relative speed between the polishing pad and thesubstrate, and the load on the substrate can cause variations in thematerial removal rate. These variations cause variations in the timeneeded to reach the polishing endpoint. Therefore, the polishingendpoint cannot be determined merely as a function of polishing time.

[0005] One way to determine the polishing endpoint is to remove thesubstrate from the polishing surface and examine it. For example, thesubstrate may be transferred to a metrology station where the thicknessof a substrate layer is measured, e.g., with a profilometer or aresistivity measurement. If the desired specifications are not met, thesubstrate is reloaded into the CMP apparatus for further processing.This is a time consuming procedure that reduces the throughput of theCMP apparatus. Alternatively, the examination might reveal that anexcessive amount of material has been removed, rendering the substrateunusable.

[0006] Several methods have been developed for in-situ polishingendpoint detection. Most of these methods involve monitoring a parameterassociated with the substrate surface, and indicating an endpoint whenthe parameter abruptly changes. For example, where an insulative ordielectric layer is being polished to expose an underlying metal layer,the coefficient of friction and the reflectivity of the substrate willchange abruptly when the metal layer is exposed.

[0007] Where the monitored parameter changes abruptly at the polishingendpoint, such endpoint detection methods are acceptable. However, asthe substrate is being polished, the polishing pad condition and theslurry composition at the pad- substrate interface may change. Suchchanges may mask the exposure of an underlying layer, or they mayimitate an endpoint condition. Additionally, such endpoint detectionmethods will not work if only planarization is being performed, if theunderlying layer is to be over-polished, or if the underlying layer andthe overlying layer have similar physical properties.

SUMMARY

[0008] A method is disclosed for determining an endpoint associated withchemical mechanical polishing a metal layer on a substrate, the endpointhaving a predetermined pattern reflected of reflected high intensity. Inone aspect, the method includes bringing a surface of the substrate intocontact with a polishing pad that has a window; causing relative motionbetween the substrate and the polishing pad; directing a light beamthrough the window, the motion of the polishing pad relative to thesubstrate causing the light beam to move in a path across the substrate;detecting light beam reflections from the metal layer; generatingreflection data associated with the light beam reflections; dividing thereflection data into a plurality of radial ranges; and identifying thepredetermined pattern from the reflection data in the plurality ofradial ranges to establish the endpoint.

[0009] Implementations of the invention includes one or more of thefollowing. The chemical mechanical polishing operation is stopped whenthe endpoint is identified. The reflection data may be stored on a mediafor subsequent analysis. Processing of the reflection data may be donein real-time or off-line. The identifying step may include comparing thereflection data to a predetermined threshold. The identifying step mayinclude determining whether the reflection data is in a downward trend,an upward trend, or a flat trend. The detecting step measuresreflections corresponding to a sampling zone in the path across thesubstrate and may include determining a radial position for eachsampling zone; determining a position of the carrier head from a carrierhead sweep profile; and dividing the reflection measurements into aplurality of radial ranges according to the radial position.

[0010] In another aspect, a method determines an endpoint associatedwith chemical mechanical polishing a metal layer, the endpoint having apredetermined pattern of reflected light intensity. The method includesdirecting a light beam through a window of a polishing pad and movingthe polishing pad relative to the substrate to cause the light beam tomove in a path across the substrate; detecting light beam reflectionsfrom the metal layer; generating reflection data associated with thelight beam reflections; dividing the reflection data into a plurality ofradial ranges; and identifying the predetermined pattern from thereflection data in the plurality of radial ranges to establish theendpoint.

[0011] Implementations of the invention include one or more of thefollowing. The chemical mechanical polishing may be stopped when theendpoint is identified. The reflection data may be stored on a media forsubsequent analysis. The identifying step includes comparing thereflection data to a predetermined threshold. The identifying step mayalso include determining whether the reflection data has a downwardtrend, an upward trend or a flat trend.

[0012] In another aspect, an apparatus for polishing a metal layer of asubstrate includes a carrier head to hold the substrate; a polishing padadapted to contact a surface of the substrate, the polishing pad havinga window therethrough; a motor coupled to the polishing pad for causingrelative motion between the substrate and the polishing pad; a lightsource to direct a light beam through the window, the motion of thepolishing pad relative to the substrate causing the light beam to movein a path across the substrate; a sensor optically coupled to the lightsource for detecting light beam reflections from the substrate, thesensor generating reflection data associated with the light beamreflections; an electronic bin coupled to the sensor for separating thereflection data into a plurality of radial ranges; and a patternrecognizer coupled to the sensor and to the bin for identifying theendpoint by comparing the predetermined pattern to the reflection data.

[0013] Implementations of the invention include one or more of thefollowing. A polishing controller may be connected to the patternrecognizer, the polishing controller stopping chemical mechanicalpolishing when the endpoint is identified. The reflection data may bestored on a media for subsequent analysis. The pattern recognizercompares the reflection data to a predetermined threshold. The patternrecognizer may determine whether the reflection data has a downwardtrend, an upward trend or a flat trend.

[0014] Advantages of the invention include one or more of the following.The reflection data from a wafer is captured using a high resolutiondata acquisition system at a relatively fine time scale, on the order ofmilliseconds. Further, reflection intensity changes during polishing arecaptured for different radial positions on the substrate. The highresolution data acquisition system provides precise time control of eachprocess step in a multi-step operation. Detailed data is available onthe progress of the metal polishing operation at different locations ofthe wafer. Additionally, parameters such as uniformity of the entirewafer and removal rate for different radial portions of the wafer aredetermined. The acquired high resolution data can be processed on-lineor off-line to adjust various variables and parameters to minimizeerosion and dishing of the surface layer. If the data is processed inreal-time, the feedback data may be used for endpoint detection or forclosed-loop control of process parameters. For instance, the polishingpressure, polishing speed, chemistry, and slurry composition may bealtered in response to the feedback data to optimize the overallpolishing performance and/or polishing quality. The reflection data isavailable for experimentation to improve the deposition process.

[0015] Other features and advantages of the invention will becomeapparent from the following description, including the drawings andclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is an exploded perspective view of a chemical mechanicalpolishing apparatus.

[0017]FIG. 2 is a side view of a chemical mechanical polishing apparatusincluding an optical reflectometer.

[0018]FIG. 3 is a simplified cross-sectional view of a substrate beingprocessed, schematically showing a laser beam impinging on andreflecting from the substrate.

[0019]FIG. 4 is a graph showing a measured reflectance trace inarbitrary intensity units (a.u.).

[0020] FIGS. 5A-5E are simplified plan views illustrating the positionof a window in a polishing pad as a platen rotates.

[0021]FIG. 6 is a flow chart of a method of determining the end-point ofthe polishing of a metal layer during CMP.

[0022]FIG. 7A is a schematic view illustrating the path of a laserbeneath the carrier head.

[0023]FIG. 7B is a graph showing a hypothetical portion of a reflectancetrace generated by a single sweep of the window beneath the carrierhead.

[0024]FIG. 8 is a schematic view illustrating the radial positions ofsampling zones from the path of the laser.

[0025]FIG. 9A is a flow chart of a method of determining the radialposition of a sampling zone.

[0026]FIG. 9B is a graph showing the time at which the laser beam passesbeneath the leading and trailing edges of the substrate as a function ofthe number of rotations of the platen.

[0027]FIG. 10 is a schematic view illustrating the calculation of theradial position of the sampling zones.

[0028]FIG. 11 is a schematic diagram of a data structure to storeintensity measurements.

[0029]FIG. 12 is a graph illustrating an overlay of several reflectancetraces taken at different times.

[0030] FIGS. 13A-13H are graphs showing the reflected intensity of themetal layer as a function of distance from the center of the substrateover a polishing period.

DETAILED DESCRIPTION

[0031] Referring to FIGS. 1 and 2, one or more substrates 10 may bepolished by a CMP apparatus 20. A description of a similar polishingapparatus 20 may be found in U.S. Pat. No. 5,738,574, the entiredisclosure of which is incorporated herein by reference. Polishingapparatus 20 includes a series of polishing stations 22 and a transferstation 23. Transfer station 23 serves multiple functions, includingreceiving individual substrates 10 from a loading apparatus (not shown),washing the substrates, loading the substrates into carrier heads,receiving the substrates from the carrier heads, washing the substratesagain, and finally, transferring the substrates back to the loadingapparatus.

[0032] Each polishing station includes a rotatable platen 24 on which isplaced a polishing pad 30. The first and second stations may include atwo-layer polishing pad with a hard durable outer surface, whereas thefinal polishing station may include a relatively soft pad. If substrate10 is an “eight-inch” (200 millimeter) or “twelve-inch” (300 millimeter)diameter disk, then the platens and polishing pads will be about twentyinches or thirty inches in diameter, respectively. Each platen 24 may beconnected to a platen drive motor (not shown). For most polishingprocesses, the platen drive motor rotates platen 24 at about thirty totwo hundred revolutions per minute, although lower or higher rotationalspeeds may be used. Each polishing station may also include a padconditioner apparatus 28 to maintain the condition of the polishing padso that it will effectively polish substrates.

[0033] Polishing pad 30 typically has a backing layer 32 which abuts thesurface of platen 24 and a covering layer 34 which is used to polishsubstrate 10. Covering layer 34 is typically harder than backing layer32. However, some pads have only a covering layer and no backing layer.Covering layer 34 may be composed of an open cell foamed polyurethane ora sheet of polyurethane with a grooved surface. Backing layer 32 may becomposed of compressed felt fibers leached with urethane. A two-layerpolishing pad, with the covering layer composed of IC-1000 and thebacking layer composed of SUBA-4, is available from Rodel, Inc., ofNewark, Del. (IC-1000 and SUBA-4 are product names of Rodel, Inc.).

[0034] A rotatable multi-head carousel 60 is supported by a center post62 and is rotated thereon about a carousel axis 64 by a carousel motorassembly (not shown). Center post 62 supports a carousel support plate66 and a cover 68. Carousel 60 includes four carrier head systems 70.Center post 62 allows the carousel motor to rotate carousel supportplate 66 and to orbit the carrier head systems and the substratesattached thereto about carousel axis 64. Three of the carrier headsystems receive and hold substrates, and polish them by pressing themagainst the polishing pads. Meanwhile, one of the carrier head systemsreceives a substrate from and delivers a substrate to transfer station23.

[0035] Each carrier head system includes a carrier or carrier head 80. Acarrier drive shaft 74 connects a carrier head rotation motor 76 (shownby the removal of one quarter of cover 68) to each carrier head 80 sothat each carrier head can independently rotate about it own axis. Thereis one carrier drive shaft and motor for each head. In addition, eachcarrier head 80 independently laterally oscillates in a radial slot 72formed in carousel support plate 66. A slider (not shown) supports eachdrive shaft in its associated radial slot. A radial drive motor (notshown) may move the slider to laterally oscillate the carrier head.

[0036] The carrier head 80 performs several mechanical functions.Generally, the carrier head holds the substrate against the polishingpad, evenly distributes a downward pressure across the back surface ofthe substrate, transfers torque from the drive shaft to the substrate,and ensures that the substrate does not slip out from beneath thecarrier head during polishing operations.

[0037] Carrier head 80 may include a flexible membrane 82 that providesa mounting surface for substrate 10, and a retaining ring 84 to retainthe substrate beneath the mounting surface. Pressurization of a chamber86 defined by flexible membrane 82 forces the substrate against thepolishing pad. Retaining ring 84 may be formed of a highly reflectivematerial, or it may be coated with a reflective layer to provide it witha reflective lower surface 88. A description of a similar carrier head80 may be found in U.S. patent application Ser. No. 08/745,679, entitleda CARRIER HEAD WITH a FLEXIBLE MEMBRANE FOR a CHEMICAL MECHANICALPOLISHING SYSTEM, filed Nov. 8, 1996, by Steven M. Zuniga et al.,assigned to the assignee of the present invention, the entire disclosureof which is incorporated herein by reference.

[0038] A slurry 38 containing a reactive agent (e.g., deionized waterfor oxide polishing) and a chemically- reactive catalyzer (e.g.,potassium hydroxide for oxide polishing) may be supplied to the surfaceof polishing pad 30 by a slurry supply port or combined slurry/rinse arm39. If polishing pad 30 is a standard pad, slurry 38 may also includeabrasive particles (e.g., silicon dioxide for oxide polishing).

[0039] In operation, the platen is rotated about its central axis 25,and the carrier head is rotated about its central axis 81 and translatedlaterally across the surface of the polishing pad.

[0040] A hole 26 is formed in platen 24 and a transparent window 36 isformed in a portion of polishing pad 30 overlying the hole. Transparentwindow 36 may be constructed as described in U.S. patent applicationSer. No. 08/689,930, entitled METHOD OF FORMING A TRANSPARENT WINDOW INA POLISHING PAD FOR A CHEMICAL MECHANICAL POLISHING APPARATUS byManoocher Birang, et al., filed Aug. 26, 1996, and assigned to theassignee of the present invention, the entire disclosure of which isincorporated herein by reference. Hole 26 and transparent window 36 arepositioned such that they have a view of substrate 10 during a portionof the platen's rotation, regardless of the translational position ofthe carrier head.

[0041] A reflectometer 40 is secured to platen 24 generally beneath hole26 and rotates with the platen. The reflectometer includes a lightsource 44 and a detector 46. The light source generates a light beam 42which propagates through transparent window 36 and slurry 38 (see FIG.3) to impinge upon the exposed surface of substrate 10. For example, thelight source 44 may be laser and the light beam 42 may be a collimatedlaser beam. The light laser beam 42 is projected from laser 44 at anangle a from an axis normal to the surface of substrate 10, i.e., at anangle a from axes 25 and 81. In addition, if the hole 26 and window 36are elongated, a beam expander (not illustrated) may be positioned inthe path of the light beam to expand the light beam along the elongatedaxis of the window. Laser 44 may operate continuously. Alternatively,the laser may be activated to generate laser beam 42 during a time whenhole 26 is generally adjacent substrate 10.

[0042] Referring to FIGS. 2 and 5A-5E, CMP apparatus 20 may include aposition sensor 160, such as an optical interrupter, to sense whenwindow 36 is near the substrate. For example, the optical interruptercould be mounted at a fixed point opposite carrier head 80. A flag 162is attached to the periphery of the platen. The point of attachment andlength of flag 162 is selected so that it interrupts the optical signalof sensor 160 from a time shortly before window 36 sweeps beneathcarrier head 80 to a time shortly thereafter. The output signal fromdetector 46 may be measured and stored while the optical signal ofsensor 160 is interrupted.

[0043] In operation, CMP apparatus 20 uses reflectometer 40 to determinethe amount of material removed from the surface of the substrate, or todetermine when the surface has become planarized. A general purposeprogrammable digital computer 48 may be connected to laser 44, detector46 and sensor 160. Computer 48 may be programmed to activate the laserwhen the substrate generally overlies the window, to store intensitymeasurements from the detector, to display the intensity measurements onan output device 49, to store the intensity measurement, to sort theintensity measurements into radial ranges, and to detect the polishingendpoint.

[0044] Referring to FIG. 3, a substrate 10 includes a silicon wafer 12and an overlying metal layer 16 disposed over an oxide or nitride layer14. The metal may be copper, tungsten, aluminum, among others. Asdifferent portions of the substrate with different reflectivities arepolished, the signal output from the detector 46 varies with time.Particularly, when the metal layer 16 has been polished away to exposethe oxide or nitride layer 14, the reflectivity of the substrate drops.The time varying output of detector 46 may be referred to as an in-situreflectance measurement trace (or more simply, a reflectance trace). Asdiscussed below, this reflectance trace may be used to determine theend-point of the metal layer polishing operation.

[0045] Referring to FIGS. 4 and 5A-5E, a measured reflectance trace witha transient intensity waveform 90 generated by polishing a metal-coatedwafer is shown. The intensity waveform 90 is generated over a relativelylong time scale (measured in seconds). Characteristic features of thewaveform include top level plateau 97, each of which is surrounded byleft and right intermediate plateau 98. One cycle of the waveform 90includes left and right intermediate level plateau 98, one of the toplevel plateau 97, and a background level 94.

[0046] The intermediate plateau 98 represent reflections from theretaining ring 84, while the top level plateau 97 represent reflectionsfrom the substrate 10. The background level represents scatteredreflections from the window and slurry. The reflection from retainingring 84 is higher than background level. As the substrate 10 is polishedand the metal layer 16 is removed to expose the underlying layer 14, theend-point waveform 90 drops toward or below the level of theintermediate plateau 98.

[0047] Referring to FIGS. 4 and 5A-5E, the large scale structure ofreflectance trace 90 can be explained by reference to the angularposition of platen 24. Initially, window 36 does not have view of thesubstrate (see FIG. 5A). Consequently, laser beam 42 is not reflectedand the intensity measured by detector 46 is a result of backgroundintensity, including reflection from slurry 38 and transparent window36. This low intensity corresponds to the background level 94. As platen24 rotates, window 36 first sweeps underneath retaining ring 84 ofcarrier head 80 (see FIG. 5B). The lower surface 88 of retaining ring 84reflects a portion of laser beam 42 into detector 46, creating anintermediate intensity measurement that corresponds to intermediateplateau 98. As window 36 sweeps beneath substrate 10 (see FIG. 5C) aportion of laser beam 42 is reflected by the substrate. In general, themetal layer of substrate 10 will have a high reflectivity, resulting intop level plateau 97 on reflectance trace 90. As the platen continues torotate, window 36 passes again beneath retaining ring 84 (see FIG. 5D).Finally, window 36 sweeps out from beneath carrier head 80 (see FIG.5E), and the detector measures a low intensity that corresponds to thebackground 94.

[0048] Computer 48 of CMP apparatus 20 may use the reflectance tracegenerated by reflectometer 40 to determine the end-point of the metallayer polishing operation. Each measurement may be performed at aplurality of radial positions. In addition, computer 48 may use theintensity measurements to determine the flatness of the substrate andthe polishing uniformity for CMP tool and process qualification asexplained below.

[0049] Referring now to FIG. 6, an end-point determining process isshown. First, several polishing parameters that will be used during theend-point determination are stored in the memory of computer 48 (step101). The polishing parameters of interest include the platen rotationrate and the carrier head sweep profile.

[0050] A metal layer on a surface of the substrate 12 is polished (step102) by bringing the surface of the substrate into contact with thepolishing pad 30 (FIG. 2). The polishing pad 30 is rotated, causingrelative motion between the substrate and the polishing pad.

[0051] Transient intensity data is monitored and collected for aplurality of sampling zones (step 104). This is done by directing alight beam generated by the reflectometer 40 through the window. Themotion of the polishing pad 30 relative to the substrate 12 causes thelight beam to move in a path across the substrate surface. Light beamreflections from the substrate 10 and the retaining ring 84 are detectedby a sensor, which generates reflection data associated with the lightbeam reflections.

[0052] The transient intensity data is displayed on a monitor (step 106)for an operator to monitor the progress of the polishing operation. Apattern recognizer is applied to the transient intensity data to detectsignal changes (step 108). The pattern recognizer may simply be athreshold detector which checks whether the intensity data has fallenbelow a predetermined threshold. Alternatively, in another embodiment, awindow logic can be applied to the data to detect a sequence of signalchanges. Three types of window logic are used to detect local maxima andminima: a window logic with a downwardly cusp to detect a downward trendin the reflection data; a window logic with an upwardly cusp to detectan upward trend in the reflection data; and a window logic with asubstantially flat line to detect that the reflection data is relativelystatic. The signal changes may be averaged. More discussion of patternrecognition algorithms for endpoint detection may be found in abovementioned U.S. patent application Ser. No. 08/689,930.

[0053] The output of the pattern recognizer is a stop signal which,along with additional feedback data, is provided to a polishercontroller (step 110). The polisher controller uses the feedback data toadjust various variables and parameters to minimize erosion and dishingof the surface layer. For instance, the polishing pressure, polishingspeed, chemistry, and slurry composition may be deployed to optimize theoverall polishing performance and/or polishing quality. The stop signalcauses the polisher controller to stop the current metal layer polishingoperation (step 112).

[0054] Concurrent with steps 106-112, the process of FIG. 6 stores thetransient intensity data onto a data storage device, e.g., a computerdisk (step 114) for subsequent processing. In brief, the intensity foreach sampling zone is determined (step 116), the radial position of eachsampling zone is calculated (step 118), and the intensity measurementsare sorted into radial ranges (step 150). The sorted intensitymeasurements are used to measure the polishing uniformity and removalrates at different radial ranges of the substrate (step 152). Each ofthese steps will be discussed in greater detail below.

[0055] Generally, the reflected intensity changes during polishing fordifferent radial positions on the substrate. The metal layer may beremoved at different rates for different portions of the substrate. Forinstance, the metal layer near the center of the substrate may beremoved last, while the metal layer near the perimeter or edge of thesubstrate may be removed first, or vice versa. The reflection data fromthe entire wafer is captured at a relatively fine time scale in theorder of milliseconds and is available for experimentation to improvethe deposition process. By analyzing the recorded data, the process canbe changed to make it faster, shorter or smoother. As can beappreciated, the stored data is useful for process research anddevelopment to optimize the process performance.

[0056] Referring to FIGS. 7A and 7B, the combined rotation of the platenand the linear sweep of the carrier head causes window 36 (and thuslaser beam 42) to sweep across the bottom surface of carrier head 80 andsubstrate 10 in a sweep path 120. As the laser beam sweeps across thesubstrate, reflectometer 40 integrates the measured intensity over asampling period, T_(sample), to generate a series of individualintensity measurements I_(a), I_(b), . . . I_(j). The sample rate F (therate at which intensity measurements are generated) of reflectometer 40is given by F=1/T_(sample). Reflectometer 40 may have a sample ratebetween about 10 and 400 Hertz (Hz), corresponding to a sampling periodbetween about 2.5 and 100 milliseconds. Specifically, reflectometer 40may have a sampling rate of about 40 Hz and a sampling period of about25 milliseconds.

[0057] Thus, each time that laser 44 is activated, reflectometer 40measures the intensity from a plurality of sampling zones 122 a-122 j.Each sampling zone corresponds to the area of the substrate over whichthe laser beam sweeps during a corresponding sampling period. Insummary, in step 106, reflectometer 40 generates a series of intensitymeasurements I_(a), I_(b), . . . I_(j) corresponding to sampling zones122 a, 122 b, . . . , 122 j.

[0058] Although FIG. 7A illustrates ten sampling zones, there could bemore or fewer zones, depending on the platen rotation rate and thesampling rate. Specifically, a lower sampling rate will result in fewer,wider sampling zones, whereas a higher sampling rate will result in agreater number of narrower sampling zones. Similarly, a lower rotationrate will result in a larger number of narrower sampling zones, whereasa higher rotation rate will result in a lower number of wider samplingzones. In addition, multiple detectors could be used to provide moresampling zones.

[0059] As shown in FIG. 7B, the intensity measurements I_(a) and I_(j)for sampling zones 122 a and 122 j, respectively, are low because window36 does not have a view of the carrier head, and consequently laser beam42 is not reflected. Sampling zones 122 b and 122 i are located beneathretaining ring 84, and therefore intensity measurements I_(b) and I_(i)will be of intermediate intensity. Sampling zones 122 c, 122 d, . . .122 h are located beneath the substrate, and consequently generaterelatively large intensity measurements I_(c), I_(d), . . . I_(h) at avariety of different radial positions across the substrate.

[0060]FIG. 12 is an overlay of several transient signal graphs 300-320.Each of the transient signal graphs 300-320 represents intensity dataover an interval associated with a sweep of the window beneath thecarrier head. For instance, the graph 300 shows the end-point databetween about 1.7 seconds to about 2.7 seconds, and the graph 320 showsthe end-point data between about 350.8 seconds and about 351.8 seconds.

[0061]FIG. 12 shows how the endpoint reflected intensity signal changesduring the polishing operation. Initially, in period 300, the metallayer on the surface of the substrate 10 is jagged. The metal layer 16has some initial topography because of the topology of the underlyingpatterned layer 14. Due to this topography, the light beam scatters whenit impinges the metal layer. As the polishing operation progresses, themetal layer becomes more planar and the reflectivity of the polishedmetal layer increases during periods 302-308. As such, the signalstrength steadily increases to a stable level. From period 310-320, asthe metal layer 16 is increasingly cleared to expose the oxide layer 14,the overall signal strength declines until the polishing operation iscompleted. Thus, in period 320, only a small trace of metal remains inthe center of the substrate 10.

[0062] When entire surface of the substrate is covered with a metallayer, such as copper, the reflection from the substrate 10 has a squareprofile. As the metal layer is removed from the edge of the substrate10, the profile of the reflection from the substrate takes on atrapezoidal shape. Eventually, when the metal layer is nearly removed bythe polishing operation, the profile of the reflection from thesubstrate 10 takes on a triangular shape.

[0063] Turning now to FIG. 8, in step 108 the radial positions R_(a),R_(b), . . . R_(j) of the corresponding sampling zones 122 a, 122 b, 122j are determined. One way to determine the radial position of a samplingzone is to calculate the position of the laser beneath the substratebased on the measurement time T_(measure) and the platen rotation rateand carrier head sweep profile. Unfortunately, the actual platenrotation rate and carrier head sweep profile may not precisely match thepolishing parameters. Therefore, a preferred method 130 of determiningthe radial positions of the sampling zones is shown in FIG. 9A. First,the time T_(sym) at which laser beam 42 passes beneath a mid-line 124(see FIG. 5C) of the substrate is determined (step 132). Then the radialpositions of the sampling zones are determined from the time differencebetween the measurement time T_(measure) and the symmetric time T_(sym)(step 134).

[0064] One method of determining the symmetry time T_(sym) is to averagethe times of the first and last large intensity measurements from eachsweep, as these intensity measurements should correspond to thesubstrate edge. However, this results in some uncertainty in T_(sym)because the position of the sampling zones beneath the substrate are notknown.

[0065] Referring to FIG. 9B, in order to compute the symmetric timeT_(sym) in step 132, computer 48 determines the first and last largeintensity measurements from sweep path 120, i.e., intensity measurementsI_(c) and I_(h), and stores the corresponding measurement times T_(lead)and T_(trail). These lead and trail times T_(lead) and T_(trail) areaccumulated on each sweep to generate a series of lead times T_(lead1),T_(lead2), T_(leadN) and trail times T_(trail1), T_(trail2), . . .T_(trailN). Computer 48 stores lead times T_(lead1), T_(lead2), . . .T_(leadN) and the associate number of platen rotations 1, 2, . . . N foreach leading spike 96. Similarly, computer 48 stores the trail timesT_(trail1), T_(trail2), . . . T_(trailN) and the associated number ofrotations 1, 2, . . . N of each trailing spike 98. Assuming that platen24 rotates at a substantially constant rate, the times T_(lead1),T_(lead2), . . . T_(leadN) form a substantially linear increasingfunction (shown by line 136). Similarly, the times T_(trail1),T_(trail2), T_(trailN) also form a substantially linear increasingfunction (shown by line 137). Computer 48 performs two least square fitsto generate two linear functions T_(lead)(n) and T_(trail)(n) asfollows:

T _(lead)(n)=a ₁+(a ₂ * n)

T _(trail)(n)=a ₃+(a ₄ * n)

[0066] where n is the number of platen rotations and a₁, a₂, a₃ and a₄are fitting coefficients calculated during the least square fit. Oncethe fitting coefficients have been calculated, the symmetry time T_(sym)at which laser beam 42 crosses mid-line 124 (shown by phantom line 138)may be calculated as follows:$T_{sym} = {\frac{a_{1} + a_{3}}{2} + {\frac{a_{2} + a_{4}}{2} \cdot n}}$

[0067] By using a least square fit over several platen rotations tocalculate the symmetry time T_(sym), uncertainty caused by thedifferences in the relative position of the sampling zone beneath theretaining ring are substantially reduced, thereby significantly reducinguncertainty in the symmetry time T_(sym).

[0068] Once computer 48 has calculated the time T_(sym) at which laserbeam 42 crosses midline 124, the radial distance R_(a), R_(b), . . .R_(j) of each sampling zone 122 a, 122 b, . . . 122 j from the center126 of the substrate are calculated in step 132. Referring to FIG. 10,the radial position may be calculated as follows:

R={square root}{square root over (d²+L²−2dLcos θ)}

[0069] where d is the distance between the center of the polishing padand the center of window 36, L is the distance from the center of thepolishing pad to the center of substrate 10, and θ is the angularposition of the window. The angular position θ of the window may becalculated as follows:

θ=f_(platen) ·2π( t _(measure) −T _(sym))

[0070] where f_(platen) is the rotational rate of the platen (in rpm).Assuming that the carrier head moves in a sinusoidal pattern, the linearposition L of the carrier head may be calculated as follows:

L=L ₀ +A·cos(ω·T _(measure))

[0071] where ω is the sweep frequency, A is the amplitude of the sweep,and L₀ is the center position of the carrier sweep.

[0072] In another embodiment, position sensor 160 could be used tocalculate the time T_(sym) when the window crosses midline 124. Assumingthat sensor 160 is positioned opposite carrier head 80, flag 162 wouldbe positioned symmetrically across from transparent window 36. Thecomputer 48 stores both the trigger time T_(start) when the flaginterrupts optical beam of the sensor, and the trigger time T_(end) whenthe flag clears the optical beam. The time T_(sym) may be calculated asthe average of T_(start) and T_(end). In yet another embodiment, theplaten and carrier head positions could be determined at each sampletime T_(a), T_(b), . . . T_(h), from optical encoders connected to theplaten drive motor and radial drive motor, respectively.

[0073] Once the radial positions R_(a), R_(b), . . . R_(m) of thesampling zones have been calculated, some of the intensity measurementmay be disregarded. If the radial position R of a sampling zone isgreater than the radius of the substrate, then the intensity measurementfor that sampling zone includes mostly radiation reflected by theretaining ring or background reflection from the window or slurry.Therefore, the intensity measurements for any sampling zone that ismostly beneath the retaining ring is ignored. This ensures that spuriousintensity measurements are not used in the calculation of the thin filmlayer reflected intensity.

[0074] After several sweeps of laser beam 42 beneath the substrate,computer 48 accumulates a set of intensity measurements I₁, I₂, . . .I_(N), each associated with a measurement time T₁, T₂, . . . T_(N), anda radial position R₁, R₂, . . . R_(N). Referring to FIG. 11, as theintensity, time, and radial position measurements are accumulated insteps 106 and 108, the time and intensity measurements are sorted intobins in a data structure 140 in step 110. Each bin is associated with aradial range of sampling zones. For example, intensity measurements forsampling zones located up to 20 mm from the center of the substrate maybe placed in a first bin 142 (see FIG. 13A) which is discussed below,intensity measurements made for sampling zones located between 20 and 30mm from the center of the substrate may be placed in a second bin 144(see FIG. 13B), intensity measurements made for sampling zones locatedbetween 30 and 40 mm from the center of the substrate may be placed in athird bin 146 (see FIG. 13C), and so on. The exact number of bins andthe radial ranges of the bins depend upon the information that the userdesires to extract. In general, the radial range of each bin may beselected so that a sufficient number of intensity measurements areaccumulated in the bin to provide visually meaningful information.

[0075] The calculations discussed above are performed for each bin,thereby providing reflected intensity measurements at a plurality ofradial positions across the surface of the substrate. Graphs of theinitial and final reflected intensity of the thin film layer as afunction of radius are shown in FIGS. 12 discussed above as well as inFIGS. 13A-13H.

[0076] Turning now to FIGS. 13A-13H, a number of traces which displayhow reflected intensity changes during polishing for different radialpositions on the substrate 10 are shown. The charts of FIGS. 13A-13Hillustrate that the metal layer is removed at different rates fordifferent portions of the substrate. Generally, FIGS. 13A-13H show thatthe metal layer near the center of the substrate is removed last, whilethe metal layer near the perimeter or edge of the substrate is clearedfirst. For example, FIG. 13A shows that the metal layer within a radiusrange of 0-20 mm is removed at about 330 seconds. FIG. 13B shows thatthe metal layer within a radius range of 20-30 mm is removed at about325 seconds. FIG. 13C shows that the metal layer within a radius rangeof 30-40 mm is removed at about 318 seconds. FIG. 13D shows that themetal layer within a radius range of 40-50 mm is removed at about 310seconds. FIG. 13E shows that the metal layer within a radius range of50-60 mm is removed at about 295 seconds. FIG. 13F shows that the metallayer within a radius range of 60-70 mm is removed at about 290 seconds.FIG. 13G shows that the metal layer within a radius range of 70-80 mm isremoved at about 290 seconds; and FIG. 13H shows that the metal layerwithin a radius range of 80-90 mm is removed as early as about 260seconds.

[0077] As shown therein, the reflectance trace for several of the radialranges exhibit two intensity levels (shown by lines 160 and 162). Thedistance between the two intensity levels increases with substrateradius. Without being limited to any particular theory, the twointensity levels may be caused by non-symmetric distribution of theslurry or the product of the reaction of the slurry and the metal layeron the substrate. Specifically, on each sweep of the laser beam acrossthe substrate, two data points are usually entered in a bin: one datapoint which is closer to the leading edge of the substrate and one datapoint which is closer to the trailing edge of the substrate. However,due to non-symmetric distribution of the slurry and the reactionproducts beneath the substrate, the laser beam may be more attenuatedwhen passing through slurry layer adjacent different regions of thesubstrate. Thus, the reflectance traces might also be used as a measureof the uniformity of slurry distribution beneath the substrate.

[0078] The reflection intensity changes during polishing are thuscaptured for different radial positions on the substrate. The highresolution data acquisition allows a precise time control of eachprocess step in a multi-step operation. A wealth of parameters such asuniformity of the entire wafer and removal rate for different radialportions of the wafer are captured. The acquired high resolution datacan be processed on-line or off-line to adjust various variables andparameters to minimize erosion and dishing of the surface layer. If thedata is processed in real-time, the real-time feedback data allows atighter closed-loop control with the process parameters. Further, thereflection data is available for process engineers to experiment withtheir processing parameters to improve the polishing process.

[0079] The present invention has been described in terms of a preferredembodiment. The invention, however, is not limited to the embodimentdepicted and described. Rather, the scope of the invention is defined bythe appended claims.

What is claimed is:
 1. A method for determining an endpoint associatedwith chemical mechanical polishing a metal layer on a substrate, theendpoint having a predetermined pattern reflected of reflected highintensity, the method comprising: bringing a surface of the substrateinto contact with a polishing pad that has a window; causing relativemotion between the substrate and the polishing pad; directing a lightbeam through the window, the motion of the polishing pad relative to thesubstrate causing the light beam to move in a path across the substrate;detecting light beam reflections from the metal layer; generatingreflection data associated with the light beam reflections; dividing thereflection data into a plurality of radial ranges; and identifying thepredetermined pattern from the reflection data in the plurality ofradial ranges to establish the endpoint.
 2. The method of claim 1 ,further comprising stopping chemical mechanical polishing when theendpoint is identified.
 3. The method of claim 1 , further comprisingstoring the reflection data on a media for subsequent analysis.
 4. Themethod of claim 3 , further comprising processing the reflection data inreal-time.
 5. The method of claim 3 , further comprising processing thereflection data off-line.
 6. The method of claim 1 , wherein theidentifying step further comprises comparing the reflection data to apredetermined threshold.
 7. The method of claim 1 , wherein theidentifying step further comprises determining whether the reflectiondata is in a downward trend.
 8. The method of claim 1 , wherein theidentifying step further comprises determining whether the reflectiondata is in an upward trend.
 9. The method of claim 1 , wherein theidentifying step further comprises determining whether the reflectiondata is in a flat trend.
 10. The method of claim 1 , wherein thedetecting step measures reflections corresponding to a sampling zone inthe path across the substrate, the method including: determining aradial position for each sampling zone; determining a position of thecarrier head from a carrier head sweep profile; and dividing thereflection measurements into a plurality of radial ranges according tothe radial position.
 11. A method for determining an endpoint associatedwith chemical mechanical polishing a metal layer, the endpoint having apredetermined pattern of reflected light intensity, the methodcomprising: directing a light beam through a window of a polishing padand moving the polishing pad relative to the substrate to cause thelight beam to move in a path across the substrate; detecting light beamreflections from the metal layer; generating reflection data associatedwith the light beam reflections; dividing the reflection data into aplurality of radial ranges; and identifying the predetermined patternfrom the reflection data in the plurality of radial ranges to establishthe endpoint.
 12. The method of claim 11 , further comprising stoppingchemical mechanical polishing when the endpoint is identified.
 13. Themethod of claim 11 , further comprising storing the reflection data on amedia for subsequent analysis.
 14. The method of claim 11 , wherein theidentifying step comprises comparing the reflection data to apredetermined threshold.
 15. The method of claim 11 , wherein theidentifying step further comprises determining whether the reflectiondata has a downward trend, an upward trend or a flat trend.
 16. Anapparatus for polishing a metal layer of a substrate, comprising: acarrier head to hold the substrate; a polishing pad adapted to contact asurface of the substrate, the polishing pad having a windowtherethrough; a motor coupled to the polishing pad for causing relativemotion between the substrate and the polishing pad; a light source todirect a light beam through the window, the motion of the polishing padrelative to the substrate causing the light beam to move in a pathacross the substrate; a sensor optically coupled to the light source fordetecting light beam reflections from the substrate, the sensorgenerating reflection data associated with the light beam reflections;an electronic bin coupled to the sensor for separating the reflectiondata into a plurality of radial ranges; and a pattern recognizer coupledto the sensor and to the bin for identifying the endpoint by comparingthe predetermined pattern to the reflection data.
 17. The apparatus ofclaim 16 , further comprising a polishing controller coupled to thepattern recognizer, the polishing controller stopping chemicalmechanical polishing when the endpoint is identified.
 18. The apparatusof claim 16 , wherein the reflection data is stored on a media forsubsequent analysis.
 19. The apparatus of claim 16 , wherein the patternrecognizer compares the reflection data to a predetermined threshold.20. The apparatus of claim 16 , wherein the pattern recognizerdetermines whether the reflection data has a downward trend, an upwardtrend or a flat trend.